1. Field of the Invention
The present invention relates to a re-imaging lens for use in a focus state detecting device, particularly adapted for use in a focus state detecting device for detecting the focusing state of an objective lens according to distribution of luminance in plural directions of the object.
2. Related Background Art
There is already proposed, as shown in FIG. 20, a focus state detecting device 2 employing a re-imaging lens, for detecting the focusing state of a photographing lens, for example of an automatic focusing single-lens reflex camera, according to the distribution of luminance in the horizontal and vertical directions of the object.
The focus state detecting device 2 is composed of a field mask 3 having a cross-shaped linear aperture 3X extended in the horizontal direction X and vertical direction Y from the optical axis LH, a condenser lens 4, a divided pupil mask 5 having four apertures 5a, 5b, 5c, 5d separated by a predetermined distance in the horizontal direction X and vertical direction Y from the optical axis LH, a re-imaging lens 6 consisting of four convex lenses 6a, 6b, 6c, 6d separated by a predetermined distance in the horizontal direction X and vertical direction Y from the optical axis LH, and photosensor unit 7 having a cross-shaped arrangement of four line sensors 7a, 7b, 7c, 7d, and the above-mentioned components are arranged in succession along the optical axis LH of the photographing lens 1.
The condenser lens 4 forms projected images 1a, 1b, 1c, 1d of the four apertures 5a, 5b, 5c, 5d of the divided pupil mask 5 positioned in front of the re-imaging lens 6, on the pupil of the photographing lens 1, as indicated by broken lines thereon. The field mask 3 is positioned approximately on the estimated focal lens of the photographing plane 1, whereby images of the field mask 3 are formed on the photosensor unit 7 by means of the four convex lenses 6a, 6b, 6c, 6d of the re-imaging lens 6.
Consequently, a ray of light coming from the object and passing through the first projected image 1a formed on the photographing lens 1 is guided through the field mask 3, condenser lens 4 and first aperture 5a of the divided pupil mask 5, and forms an unsharp or sharp image of the object on the first line sensor 7a of the photosensor unit 7 by means of the first convex lens 6a of the re-imaging lens 6.
On the other hand, a ray coming from the object and passing through the second projected image 1b formed on the photographing lens 1 is guided through the field mask 3, condenser lens 4 and second aperture 5b of the divided pupil mask 5, and similarly forms an image of the object on the second line sensor 7b of the photosensor unit 7, by means of the second convex lens 6b of the re-imaging lens 6.
Also with respect to the luminance distribution in the vertical direction of the object, the focusing state of the photographing lens 1 is detected from the positions of a secondary image formed on the third line sensor 7c by the ray coming from the object and passing through the third projected image 1c on the photographing lens 1 and of a secondary image formed on the fourth line sensor 7d by the ray coming from the object and passing through the fourth projected image 1d on the photographing lens 1.
In such focus state detecting device of the above-explained principle, there has been employed a re-imaging lens in which first and second convex lenses 6a, 6b of flat circular shape are positioned at an equal distance in the horizontal direction X from the optical axis LH, and third and fourth convex lenses 6c, 6d of flat circular shape are positioned at an equal distance in the vertical direction Y from the optical axis.
In such re-imaging lens 6, the first to fourth convex lenses 6a-6d are so positioned as not to contact one another, so that the sum of radii of two neighboring lenses cannot be made larger than the distance between the centers thereof. Consequently the effective areas of the convex lenses 6a-6d are relatively limited, and the light from the object, obtained through the limited projected image areas 1a-1d on the photographing lens 1 cannot be efficiently utilized.
Also there has been proposed a structure, as shown in FIGS. 21-23, having convex lenses 11a, 11b, 11c, 11d corresponding to those 6a-6d in such a manner that the first and second convex lenses 11a, 11b positioned at the same distances across the optical axis LH along the horizontal direction X overlap each other, and the third and fourth convex lenses 11c, 11d in the vertical direction Y similarly overlap each other, and that the boundaries of said convex lenses 11a, 11b, 11c, 11d run mutually orthogonally on the plane of the lens and converging on the optical axis LH.
However, such re-imaging lens 11 is not suitable if the detecting optical system for the horizontal luminance distribution is designed differently from that for the vertical luminance distribution, since the horizontal pair of convex lenses 11a, 11b is identical with the vertical pair 11c, 11d.